The present invention concerns apparatus for use in a well logging environment, and more particularly, with such apparatus for use in high or elevated temperature environments in a borehole in earth formations.
Generally speaking in recent years, wells have been drilled to greater and greater depths to reach petroleum or hydrocarbon deposits than in the past. Because of the greater depths to which wells have been drilled, higher temperature formations and high borehole temperatures have been encountered. Also in recent years, interest in so-called geothermal wells has increased substantially. The geothermal wells embody wells into which high temperature steam generated in the earth's crust is transported to the surface for use in driving electrical generators or the like. In general, geothermal wells have a higher temperature than would be encountered in a hydrocarbon producing well at the same depth. Because of the substantially higher temperatures encountered in deeper hydrocarbon producing wells and in geothermal wells, well logging service companies have been called upon to provide electronically powered and controlled well logging tools which operate at increasingly higher temperature and more hostile environments than in the past.
The construction of electronic measurement systems for well logging use at higher temperature ranges has typically followed two paths. The first path has been to design electronic circuits using higher temperature rated components than were previously used. Unfortunately, the higher the temperature rating requirement on electronic components, the fewer the number of commercially available components. Thus, the circuit design becomes limited by the available variety of parts having high temperature ratings rather than by the design concepts for the electronic circuits. The second path which has been followed is to design circuits which have limited temperature ratings and then to package these circuits within an environment capable of protecting them from the elevated temperatures encountered in operation. For example, such packaging techniques have included the use of an insulated vacuum flask similar to a conventional thermos bottle. Such systems have included within the flask a material for heat absorption from the electronically powered circuits contained in the thermos or vacuum flask. Typically a material used for this purpose has been a bismuth alloy of a nature eutectic which changes phase from solid to liquid at approximately 281.degree. F. and in this phase change, absorbs a substantial amount of heat energy according to its specific heat of fusion. A eutectic alloy has a melting temperature which coincides with its freezing temperature, thus having no freezing range between its solid and liquid states. A typical alloy suitable for such use is "Cerrotru (.RTM.)" (registered trademark of the Cerro Corp. of New York, N.Y.). This alloy has a latent heat of fusion 20 BTU/LB. and comprises an alloy of about 58% bismuth and 42 percent tin.
In high temperature logging operations beginning at a surface ambient temperature, a well logging tool is rapidly lowered into a high temperature region of the borehole and logging measurements are made as rapidly as possible. As the temperature within the electronic package rises due to external heat flow and the internal power dissipation of the electronic components, the temperature within the package can be somewhat limited or modified by the heat absorbant alloy material. This process works until all the alloy has changed phase from a solid to a liquid employing its latent heat of fusion. The maximum time that the well logging instrument may remain in the hostile temperature environment is thus limited by the amount of heat absorbant material available and additionally by the efficiency of placement of the heat absorbant material to absorb heat from the electronic componentry which generates internal heat due to its consumption of electrical power.
In typical construction schemes used in the prior art the heat absorbing material has been placed in a vacuum flask adjacent electronic circuit components. Typically such placement has been at one end of the vacuum flask, the remainder of the flask being employed to contain the electronic circuit modules. In such construction techniques, the heat transfer mechanism throughout the vacuum flask is restricted by the packaging system. Flow of air across the electronic components is virtually nonexistant due to transverse wafers which are reinforcing members used to support the electronic circuitry and to reinforce the internal walls of the vacuum flask. The temperature between individual spacers or modules of the electronic componentry may rise considerably above the temperature of the heat absorptant material situated at one end of the vacuum flask due to high power dissipation in some electronic components. Primarily heat transfer by conduction through metallic members is relied upon, since there is very poor air circulation to conduct the heat from the components generating power dissipation to the heat absorbant material within the vacuum flask. Moreover, the entire length of the vacuum flask, and hence the well logging tool, utilizing such a packaging system must be increased approximately 50 percent under prior art conditions, in order to provide space for the heat absorbent material within the instrument housing. For example, there must be approximately 12 inches of heat absorbing material for each 24 inches of circuit components when placed end to end in the manner known in the prior art. The heat absorber thus increases the length requirement of the well logging instrument housing by approximately 50 percent.